The first month following birth is the window in which humans are at their greatest risk of death. A million babies die on the day of their birth every year. But if they receive exactly the same level of care, females are statistically less likely to die than males. Lawn's research encompasses data from across the globe, giving the broadest picture possible of infant mortality. And having researched the issue in such depth, Lawn concludes that boys are at around a 10 percent greater risk than girls in that first month—and this is at least partly, if not wholly, for biological reasons.

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"If you have parity in your survival rates, it means that you aren't looking after girls," says Lawn. "The biological risk is against the boy, but the social risk is against the girl.